说明：  锁相环和锁频环在数字costas环中的应用.pdf 一篇关于costa环路的新颖设计方案，包含大量的仿真图和性能分析，对学习锁相环有很大帮助
(And frequency-locked loop PLL digital costas loop in the application. Pdf a novel about the costa loop design, contains a large number of simulation map and performance analysis are very helpful for learning PLL)